1963 Zenith 29JC20 restoration

[DaveWM]

maybe some delay line ringing?

[Zenith6S321]

Originally Posted by DaveWM

maybe some delay line ringing?

I just tried adjusting the delay line termination potentiometer. It did not affect this ringing.

Post #44 shows pictures of test pattern video driven into the first IF showing ringing and then composite video driven into just after the video detector that does not show nearly as much ringing. Sure seems like it's in the IF stages.

Dave

[DaveWM]

oh my yes, I missed that. I wonder if all the little ferrite beads are in place etc. on the filament leads? My later zenith (24MC32) has several IIRC on the IF filaments. Mine also has a 22k resistor across the secondary of the IF before the 3rd IF tube. Wonder if maybe that has drifted high? I assume its there to damp out oscillation.

[Zenith6S321]

Originally Posted by DaveWM

oh my yes, I missed that. I wonder if all the little ferrite beads are in place etc. on the filament leads? My later zenith (24MC32) has several IIRC on the IF filaments. Mine also has a 22k resistor across the secondary of the IF before the 3rd IF tube. Wonder if maybe that has drifted high? I assume its there to damp out oscillation.

My first two IF stages filaments have a ferrite bead, as well as a couple more beads in the filaments some color circuit tubes. I did check and replace out of tolerance resistors in the IF stages, being careful to match lead dress as close to original as possible.

[Zenith6S321]

Before I started to do an IF alignment, I took some pictures of some video test pattern waveforms taken at the input to the first video amp. All the tuner and IF tubes have been tested and swapped with not much difference in the ringing. I used my Sencore VA62 to generate a channel 3 RF signal displaying the single vertical and horizontal line cross. The first video amp input scope picture shows the ringing. I then set the VA62 to generate its 10 section multi-burst which is a set of white and black vertical lines of increasing frequency from 0 to 4.5 MHz. The second picture shows the poor frequency response.

[Zenith6S321]

Here is a picture of the SAMs IF frequency response. Figure 1 is the third IF and figure 2 is the first and second response. The next picture shows the third IF response is pretty close to what SAMs shows (you have flip the plot horizontally to match my scope). The next picture is what I got for the first and second IF response. Its way off. The response at the 45.75 MHz marker shown should be much lower amplitude. Following SAMs alignment I tried to adjust A6, the 41.25 MHz trap and found it very hard to turn because its core was coming apart. I pulled the chassis, removed the coil, and removed the core. One end had two small pieces broken off. I tried a shorter ferrite core to see if the coil form was too tight. It was very tight. I ended up finding that a 1/4"-28 tap matches this ferrite core. So I tapped the coil form as shown in the next picture. The last picture shows the shavings removed from the coil form. The small ferrite core now threads nicely though the core. I am trying to glue the two small ferrite pieces back onto the broken core. If they hold, I will invert the core so that the broken pieces will be at the end that is not usually used for adjusting. Anyone know of a place to buy 1/4" threaded ferrite cores?

Dave

[Findm-Keepm]

Originally Posted by Zenith6S321

Here is a picture of the SAMs IF frequency response. Figure 1 is the third IF and figure 2 is the first and second response. The next picture shows the third IF response is pretty close to what SAMs shows (you have flip the plot horizontally to match my scope). The next picture is what I got for the first and second IF response. Its way off. The response at the 45.75 MHz marker shown should be much lower amplitude. Following SAMs alignment I tried to adjust A6, the 41.25 MHz trap and found it very hard to turn because its core was coming apart. I pulled the chassis, removed the coil, and removed the core. One end had two small pieces broken off. I tried a shorter ferrite core to see if the coil form was too tight. It was very tight. I ended up finding that a 1/4"-28 tap matches this ferrite core. So I tapped the coil form as shown in the next picture. The last picture shows the shavings removed from the coil form. The small ferrite core now threads nicely though the core. I am trying to glue the two small ferrite pieces back onto the broken core. If they hold, I will invert the core so that the broken pieces will be at the end that is not usually used for adjusting. Anyone know of a place to buy 1/4" threaded ferrite cores?

Dave

I've got a bunch (~ half of a cigar box full) of ferrite cores - how long?

I always pulled the cores from sets I junked - and at my dad's shop, I was the TV stripper/parts puller/gofer. I probably have junked over 300 sets in 30+ years. First we saved everything, then just certain chassis, then just modules, and finally, only good chassis/CRTs. My user name comes from my attitude - if you find it, keep it. Someone, somewhere will likely need it.

Cheers,